Answer by Joel Coehoorn for How do I get the difference between two Dates in JavaScript?

If you don't care about the time component, you can use .getDate() and .setDate() to just set the date part.

So to set your end date to 2 weeks after your start date, do something like this:

function GetEndDate(startDate){    var endDate = new Date(startDate.getTime());    endDate.setDate(endDate.getDate()+14);    return endDate;}

To return the difference (in days) between two dates, do this:

function GetDateDiff(startDate, endDate){    return endDate.getDate() - startDate.getDate();}

Finally, let's modify the first function so it can take the value returned by 2nd as a parameter:

function GetEndDate(startDate, days){    var endDate = new Date(startDate.getTime());    endDate.setDate(endDate.getDate() + days);    return endDate;}